Serialize for MultiSearchResponse

[milesgranger]

What does this PR do?

This adds serde::Serialize to MultiSearchResponse.

I'm in a situation where I proxy requests between client and Meilisearch. During which I have authentication via OAuth and although client has logic in place, I want to inspect responses from Meilisearch and ensure the authenticated user is only getting data they are authorized for given group associations, whether indices or specific records in other index hits.

The knee-jerk suggestion is to use different API keys. However, we have indices where for different users they have different permissions on what they can see and it's just not feasible/practical to duplicate indices/records into every combination of access rights.

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• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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Summary by CodeRabbit

• New Features
  • Improved support for exporting search results and related data in serialized formats.